1. Field of the Invention
The present invention relates to a vertical MOS type semiconductor device and, more particularly, to an IGBT (Insulated Gate Bipolar Transistor) with a current sensing function which is constituted of a plurality of principal current cells and at least one current sensing cell.
2. Description of the Related Art
In a prior art IGBT having a current sensing function, an external resistor is connected between a principal electrode and a current sensing electrode. The IGBT generally senses a current by detecting a difference in potential between the electrodes.
FIG. 1 illustrates the structure of the prior art IGBT having a current sensing function. As illustrated in FIG. 1, a plurality of gate oxide films 102 are formed on one major surface of an N-type semiconductor layer 101, and a plurality of gate electrodes 103 are arranged on their respective gate oxide films 102. Each of P-type base regions 104 is formed between two gate electrodes 103 in the surface of the N-type semiconductor layer 101. Emitter regions 105 are provided in the surface portion of each of the P-type base regions 104. A P-type semiconductor layer 106 serving as a collector region is formed on the other major surface of the N-type semiconductor layer 101. Interlayer insulation films 107 are provided so as to cover the gate electrodes 103, and a principal electrode 108 and a current sensing electrode 109 are formed on the interlayer insulation films 107 and the one major surface of the N-type semiconductor substrate 101, thus completing a principal current cell region 110 and a current sensing cell region 111 arranged adjacent to each other.
In the IGBT described above, the width of the gate electrode 103 in each cell is constant, as is the interval between the gate electrodes 103. When a positive bias is applied to the gate electrode 103 in a normal current sensing state, the channel of a MOS gate (P-type base region 104 under the gate electrode 103) is inverted to an N type and thus the adjacent principal current cell region and current sensing cell region are connected to each other through an N drift region. Therefore, part of the current in the current sensing cell region 111, which is to flow through the external resistor (not shown), is caused to flow into the principal current cell region 110 through an internal parasitic resistor (resistive component RN.sup.- of the N-type semiconductor layer 101).
Since the above parasitic resistor is a bulk resistor formed by silicon, it has its own temperature characteristics and increases in resistance as the temperature rises, thus causing a problem wherein the current flowing through the external resistor increases as the temperature rises and the detected voltage (difference in potential between external resistors) heightens. In FIG. 1, Rch indicates a resistive component of the channel.
In order to resolve the above problem, an IGBT having a structure capable of lessening the influence of temperature characteristics of the parasitic resistor is contrived.
FIG. 2 illustrates an IGBT which is so constituted that the value of a parasitic resistor is larger than that of an external resistor in order to lessen the influence of temperature characteristics of the parasitic resistor. In the IGBT, an interval a between adjacent P-type base regions 104 of the principal current cell 110 and current sensing cell 111, is greater than an interval b between other adjacent P-type base regions 104. More specifically, the width of a gate electrode 103 between the cells 110 and 111 is set greater than that of another gate electrode 103 (an interval between adjacent gate electrodes 103 is fixed), and the width of an N drift region connecting the adjacent cells 110 and 111 is increased. Thus, the value of a parasitic resistor (resistive component RN.sup.- of N-type semiconductor layer 101) is increased to improve the temperature characteristics.
However, the temperature characteristics of the parasitic resistor are slightly improved. Since, moreover, the curvature of a depletion layer 101' is acute in an area of the layer 101' where the interval a between the P-type base regions 104 is increased, a breakdown voltage is lowered. (See B. J. Baliga, "Modern Power Device", pp.272 to pp.274, for example.)